I got confused by the GHC documentation recently, I was wondering how it could be improved. From: http://www.haskell.org/ghc/docs/latest/html/users_guide/bang-patterns.html
A bang only really has an effect if it precedes a variable or wild-card pattern: f3 !(x,y) = [x,y] f4 (x,y) = [x,y] Here, f3 and f4 are identical; putting a bang before a pattern that forces evaluation anyway does nothing.
The first sentence is true, but only in settings where the pattern is being evaluated eagerly -- the bang in:
f3 a = let !(x,y) = a in [1,x,y] f4 a = let (x,y) = a in [1,x,y] has an effect.
The first time I read this, I took the first sentence to be a unqualified truth and ended up thinking that !(x,y) was equivalent to (x,y) everywhere. Stuff that comes later actually clarifies this, but I missed it. What about making the distinction clear upfront? Something like:
A bang in an eager pattern match only really has an effect if it precedes a variable or wild-card pattern: f3 !(x,y) = [x,y] f4 (x,y) = [x,y] Because f4 _|_ will force the evaluation of the pattern match anyway, f3 and f4 are identical; the bang does nothing.
It also might be a good idea to immediately follow this with the let/where usage:
A bang can also preceed a let/where binding to make the pattern match strict. For example: let ![x,y] = e in b is a strict pattern... (in the existing docs, let comes a bit later):
Just a thought. Hopefully someone can come up with a better way of wording what I'm getting at. Thanks, -Brian _________________________________________________________________ Windows Live™ Hotmail®…more than just e-mail. http://windowslive.com/howitworks?ocid=TXT_TAGLM_WL_t2_hm_justgotbetter_howi...